Mobile Ionic Contamination (MIC)

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Mobile Ionic Contamination

Mobile Ionic Contamination (MIC) refers to the presence of ionic contaminants in the active circuitries of semiconductor devices, the most common of which are alkali ions such as Na⁺, K⁺, and Cl^-. It is usually observed in gate oxide layers of MOS transistors. These contaminant ions are free to move about, hence the phrase 'mobile ionic contamination.' This mobility is accelerated by temperature and electric field.

The mobile ions often enter the gate oxide through the interface between the gate (usually metal or polysilicon) and the gate oxide (usually SiO₂). Some of the ions then drift to the Si-SiO₂ interface under the influence of electric fields created by voltages applied to the gate. Given the high mobility of these ions in SiO₂, they can drift under field assistance even at room temperature.

The presence of these ionic contaminants at the gate-oxide and oxide-semiconductor interfaces and in the oxide itself results in a mobile ionic charge, Qm, which can cause long-term changes in the threshold voltage, V_T, of the transistor. The V_T shift aggravates as more charges accumulate at the Si/SiO₂ interface.

According to S. Wolf and R. N. Tauber, a Qm value in the low 10¹⁰/cm²range will cause a shift of only a few tenths of a volt for a MOS device with a gate oxide of 1000 angstroms. However, a Qm value in the 10¹⁰/cm² range can result in V_T shifts of several volts. Thus, reducing Qm density should be a key ingredient of any program designed to eliminate MIC failures.

A high Qm value can also promote the formation of conducting channels that increase leakage currents. Gain reduction due to mobile ionic contamination has likewise been observed. Bipolar devices can also experience beta degradation due to the presence of mobile ionic contaminants, mainly because these can change carrier concentrations.

Figure 1. The mobile ionic contaminants present in the gate oxide can

accumulate as an ionic charge that affects the channel of a MOS transistor.

Among the common contaminants, Na⁺ exhibits the greatest mobility due to its small atomic radius. It is also usually the first mobile ionic contaminant to suspect if MIC is being dealt with because Na⁺ is widely distributed, being present in air and in human byproducts such as perspiration and saliva.

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